The present invention relates to a holographic recording and reconstructing apparatus and method for recording and reconstructing holograms as data.
The development of holographic recording apparatuses for recording data using holography is advancing.
In the holographic recording apparatus, one laser beam is divided into two beams, i.e., a signal beam that is modulated (data-superimposed) and a reference beam that is unmodulated. Those beams are applied to the same position in a holographic recording medium. Consequently, the signal beam interferes with the reference beam on the holographic recording medium to form a diffraction grating (hologram) in the beam-irradiated position, thus recording data on the holographic recording medium.
When the reference beam is applied to the recorded medium, a diffracted beam (reconstructed beam) is generated from the diffraction grating formed during recording. The reconstructed beam includes data superimposed on the signal beam during recording. A photodetector receives the reconstructed beam to output signals corresponding to the recorded data, so that the data can be read.
In some cases, in order to record many pieces of information on a holographic recording medium, many holograms are formed on the holographic recording medium. In this case, holograms are not always formed in different positions on the holographic recording medium. Holograms can be formed in the same position (alternatively, overlapping areas) on the holographic recording medium. This is called multiplexed recording. There are various multiplexing methods for holographic recording, e.g., angular multiplexing, wavelength multiplexing, rotation multiplexing, and shift multiplexing.
For example, in the angular multiplexing method, the incident angle of a reference beam is being shifted every recording, so that holograms are formed in the same position in a holographic recording medium using the reference beams with shifted incident angles. A reconstructed beam corresponding to each hologram formed in the same position, i.e., data can be obtained by using the corresponding reference beam that is the same as that used during recording.
In the shift multiplexing method, holograms are recorded on a holographic recording medium while a beam irradiated position is being shifted (in the lateral direction) by a distance smaller than the size of a hologram pattern formed on the holographic recording medium.
In a coaxial holographic method, a signal beam pattern and a reference beam pattern are coaxially arranged such that the reference beam pattern surrounds the signal beam pattern, so that the signal beam interferes with the reference beam in all directions to record a hologram.
The development of holographic recording apparatuses using phase correlation multiplexing, a kind of multiplexed recording methods, is advancing. Those apparatuses intend to increase the storage capacity of a holographic recording medium. Japanese Unexamined Patent Application Publication No. 11-242424 discloses an example of this kind of apparatus.
As holographic recording media, attention is being paid to photopolymer media because the manufacturing cost is low, the durability is high, and the sensitivity is high. However, such a photopolymer holographic recording medium has a phenomenon in which the angle or spacing of a diffraction grating varies due to a change in the dimension of the medium, such as the shrinkage on polymerization during recording or the shrinkage or expansion of the polymer with temperature change. The variation in the geometry of the diffraction grating leads to the deviation of an angle for peak diffraction efficiency during reconstruction from that during recording. The reliability of reconstruction may be reduced.
In the angular multiplexing method, one approach to solving the above-described problem is to shift the angle of the grating during reconstruction so as to obtain the peak diffraction efficiency. However, shifting the angle thereof during reconstruction from that during recording complicates recording and reconstruction processes.
In the shift multiplexing method, similarly, the variation in the geometry of the diffraction grating causes the deviation of a position for peak diffraction efficiency during reconstruction from that during recording. In this case, one approach to correcting the deviation is to shift the position during reconstruction so as to obtain the peak diffraction efficiency in a manner similar to the above approach for the angular multiplexing method. Assuming the use of a medium providing grooves indicative of recording positions, e.g., an optical disk, such as a DVD, after recording based on a predetermined groove pitch, a groove pitch to obtain the peak diffraction efficiency during reconstruction is deviated from that during recording. Disadvantageously, it results in complicated access during reconstruction.
In consideration of the above-described disadvantages, it is desirable to provide a holographic recording and reconstructing apparatus and method capable of counteracting the effects of a change in the dimension of a holographic recording medium on reconstruction, the change being caused by temperature change.